'spare time to' is correct and is usable in written English.
You would use it to talk about when you have extra time to do something. For example: I am going to use my spare time to study for my upcoming exams.
Most workers could not spare time to have children vaccinated timely and could not be involved in appropriate health promotion activities aiming at improving child's vaccination coverage.
Dr. Garnett devotes his spare time to golf-pathology research.
He put his spare time to good use.
She paints watercolors in her spare time to keep herself from going crazy.
If not, you have to devote your spare time to studying and take the examination again.
He devoted much of his spare time to swimming and to boating expeditions on the Seine.
Rifkind's confession that he had a lot of spare time to read and walk sounded appalling.
He devoted much of his spare time to revising and enlarging the Lexicon.
"Women never have spare time to gather together," Mr. Caballero said with a smile.
Once upon a time teachers had spare time to spend with their families, friends and hobbies.
I hope you can spare time to make your feelings known.
